An example of a lawn trimmer is disclosed in U.S. Pat. No. 4,126,928, which comprises a drive motor which is a gasoline engine, a tool head which is a rotating cutter tool including a rotary cutter having a tool shaft or mandrel, a frame tube in the shape of a hollow operating pole having a straight portion toward a proximal end which is connected to the drive motor and a bent portion toward a distal end which is connected to the tool head, and a flexible drive shaft rotatably inserted through the frame tube and connecting the drive motor to the tool shaft of the rotary cutter. The tool shaft of the rotary cutter is rotatably supported by means of a ball bearing within the tool head.
With the lawn trimmer mentioned above, the vibration caused by the rotation of the rotary cutter when trimming the lawn or cutting the grass will be transmitted to the hands of the operator who is holding the frame tube, which vibration of the frame tube would more or less fatigue the operator. Particularly in the case of a rotary cutter having a cutting line made of, for example, a nylon filament or the like as the cutting tool, the cutting line is likely to be frayed or broken due to abrasion resulting in the imbalance of the rotary cutter with respect to its tool shaft, consequently increasing the vibration of the frame tube. Such has been a drawback with the conventional lawn trimmer.
In order to remove the above-mentioned drawback from the conventional lawn trimmer, the lawn trimmer of the above U.S. patent employs an inner tube supported by several bushings coaxially interposed inside the frame tube, the drive shaft being inserted through the inner tube, so that the vibration of the drive shaft should be suppressed via the bushings. The above-mentioned vibration suppressing structure, however, has limitations in suppressing the vibrations of the frame tube where the vibration of the rotary cutter, i.e. the source of vibration is considerably strong.
A vegetation cutter disclosed in unexamined Japanese patent publication No. 2010-148484 comprises a frame tube having a proximal end and a distal end, an internal combustion engine connected to the proximal end of the frame tube, a cutting tool connected to the distal end of the frame tube, and a drive shaft inserted through the frame tube and driven by the engine. To the distal end of the frame tube is provided a gear case, in which a smaller gear formed at a tip end of the drive shaft is in meshing engagement with a larger gear formed at a proximal end of a rotary tool shaft of a cutting blade so that the rotation of the drive shaft is transmitted to the tool shaft to rotate the cutting blade of the cutting tool.
The disclosed vegetation cutter comprises an elastic member interposed between the drive shaft and the smaller gear and another elastic member interposed between the larger gear and the tool shaft, which configuration performs advantageous functions of suppressing vibrations produced by the torsional deformation of the drive shaft, suppressing torque variations to be transmitted to the drive shaft when impacts are given to the cutting blade, and suppressing gear noises produced by the meshing gears.
With a lawn trimmer and a vegetation cutter as mentioned above, however, some part of the cutting blade may be broken while cutting grasses or branches and the cutting blade may get imbalanced with respect to the tool shaft, which may enlarge the vibrations of the frame tube while the cutting blade is rotating. This type of vibrations due to the imbalance of the cutting blade will not be suppressed by the elastic members interposed between the drive shaft and the smaller gear and between the larger gear and the rotary shaft, and consequently the vibrations will fatigue the operator of the apparatus.